I recently purchased an old Nikkor 135 f/2.8 non-AI lens. Beautiful lens, but the closest focal distance is about five feet.

I noticed that it will 'focus' at about 4 inches as well. If I hold the lens a few inches from an object and look into the lens (no camera) an in-focus image is formed. I can use this for inspection of small parts, like a magnifier, and the image is upright, not inverted. I can magnify the subject quite a bit by increasing object–lens distance.

My 50mm f/1.8 AI lens is very different — it forms inverted images under those conditions.

Why are these different, and could I use this close focus mode in a camera somehow for macro work?

2 Answers
2

When you have the lens mounted on the camera and you look through the viewfinder you are viewing an image projected onto the camera's focussing screen. This is like taking a picture of a picture (with your eye being the second camera).

When taking a picture of a picture in this way the focusing ability of the second camera cannot affect the image itself i.e. if you photograph a non-macro image with a macro lens you can't make the image be focussed any closer.

Now when you hold the lens up to your eye you are producing a compound lens system with the camera being the first lens, and the lens in your eye being the second. When chaining lenses in this way both lenses affect the focusing properties of the whole system, thus you are able to focus much closer than when the lens is on the camera.

Whether you get an upside down image when looking through a lens depends on subject distance and where you eye is in relation to the lens. With a 50mm lens I can get a right way up image (that my eye is unable to quite bring into focus) when I hold the lens right up to my eye, and a well focussed upside down image when I hold the lens at arms length. With a longer focal length (and different construction, the 135 is a telephoto) these distances change.

The non-inverted image is virtual; it's the result of either the object or the viewer being closer to the optical center of the lens than the focal point. A virtual image can't be recorded directly; you need to use a system (your eye or a camera with a lens) capable of projecting a real image onto the recording medium. With freelensing, you're always working with a real (inverted) image.
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user2719Jan 25 '13 at 13:38